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7.2 Network Generation
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equations of mass balance. Under different saturation states, the flow equations
are first solved and the resulting pore scale velocities are then used to simulate
reactive solute transport.
The main objective of this study is a better understanding of transport of
adsorptive/reactive solutes under unsaturated conditions. Although various
mechanisms, such adsorption to the aws common line or moving AW interfaces
have been suggested to affect the adsorptive transport, the exact role of
these processes are still unclear. In this study, adsorption over a wide range
of saturations was considered by taking into account adsorption to both AW
and SW interfaces. Regions of the pore space for which a wetting film of water
coats the surface remain water-wet, as do the corners of the pore space
where water still resides; however, we neglect adsorptive to interfaces associated
with the water films. We have introduced a new formulation of adsorptive
solute transport within a pore network which helps to capture the effect of limited
mixing and adsorption under partially-saturated conditions. In contrast
to former pore-network studies, which assign one (average) pressure and one
(average) concentration to each pore element, we discretize an individual pore
space into separate smaller domains, each with its own flow rate and solute
concentration, in order to increase the accuracy of simulations. Thus, fluid
fluxes along edges of each pore are calculated and taken into account in the
simulation of adsorptive solute transport.
In this paper, after construction of a Multi-Directional Pore-Network (MDPN)
model, quasi-static drainage simulations are performed to determine pore-level
distribution of each fluid phase. Then, steady-state flow is established and
equations of mass balance for adsorptive solutes are solved to calculate transport
properties of such a distribution and to obtain BTCs of solute concentration.
Adsorption to both air-water (AW) and solid-water (SW) interfaces is
modeled. These adsorption processes are independent of each other and each
of them can have its own distribution coefficient and adsorbing area.
7.2 Network Generation
7.2.1 Pore size distributions
In the present study, the pore structure is represented using a 3D MDPN
model. Pore-body radii are assigned from a lognormal distribution, with no
spatial correlation, explained in Section 6.2.
Figure (7.1) shows the bore body size distribution used within the MDPN. Pore
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